Pyroelectric generator



June 7, 1966 c. KOLN ETAL 3,

PYROELECTRIC GENERATOR Original Filed March 3, 1961 HoT GASJ 78 78SOURCE sob INVENTOR CAROL KOLM BY PETER H. FOWLER United States Patent3,255,401 PYROELECTRIC GENERATOR Carol Koln, Bolton, and Peter H.Fowler, Watertown, Massr (both U.S. Sonics Corp., 63 RogersSL, Cambridge42, Mass.) Original application Mar. 3, 1961, Ser. No. 93,237.

Divided and this application Sept. 1, 1964, Ser. No.

8 Claims. (Cl. 322-2) of our copending application Serial No. 93,237,filed March 3, 1961, now US. Patent No. 3,198,969.

Our invention is of particular use in cases where power for conventionalelectromechanical generators is unavailable or the use of suchgenerators is commercially or otherwise impractical. In many such casesthere is a source of heat or, more accurately, atemperature differential occasioned by the presence of a heat source and a suitableheat sink capable of absorbing heat from the source. This conditionpermits the use of various types of heat engines, some of which arecapable of converting the temperature differential directly intoelectrical energy. However, prior to the present invention, thesedevices have been characterized by such deficiencies as large size, lowefliciency of conversion of heat into electricity and excessive cost.

Accordingly, it is a principal object of our invention to provide animproved electrical generating system adapted to convert heat directlyinto a usable amount of electrical energy. For example, in someapplications, it is desirable to use the electrical energy from thesystem to run wireless transmitters and receivers.

Another object of the invention is to provide a generating system of theabove type characterized by light weight and relatively high conversionefiiciency, thereby rendering it suitable for airborne and spaceapplications.

A further object of the invention is to provide a generating system ofthe above type having a relatively low cost per unit of powercapability.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the features of construction,combinations of elements and arrangements of parts which will beexemplified in the constructions hereinafter set forth and the scope ofthe invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawing, in which:

FIGURE 1 is a longitudinal section of a generator incorporating theprinciples of our invention, with some of the elements representedschematically, and

FIGURE 2 is a sectional view of a pyroelectric cylinder which may beused in the generator of FIGURE 1.

In general, our invention makes use of piezoelectric transducers,preferably of the ceramic type widely used as electroacousticaltransducers. A transducer of this type has a strong remanent electricalpolarization, and changes in dimension along the axis of polarizationresult in the development of a voltage between electrodes intersected bythis axis. Since the dimensions of the transducer change withtemperature, in common with most materials, a piezoelectric voltage isdeveloped whenever the temperature of the transducer varies.

Also, there are strictly pyroelectric effects associated 3,255,401Patented June 7, 1966 ice with transducers of this type. That is, avoltage is developed when the temperature is changed, even thoughchanges in dimension are blocked. This is known as the primarypyroelectric effect. The voltage produced by the piezoelectric action istermed the secondary effect. There is also a tertiary effect, thedevelopment of a voltage proportional to the rate of change oftemperature or the temperature gradient.

All three pyroelectric effects develop voltages Whose polarity dependson whether the temperature is increased or decreased. More specifically,when the temperature is increasing, With heat flow in the direction ofthe polarization, the voltages developed by all three effects are of thesame polarity. We have made use of the pyroelectric effects bycyclically exposing the transducers to the elevated temperature of aheat source and then to the lower temperature of a heat sink. Thevoltages developed between the transducer electrodes are applied to thee1ectrical load which is to consume the generated power.

In FIGURE 1 we have illustrated a generator embodying our invention inwhich a pyroelectric cylinder 64 is concentric with an outer heat sinkgenerally indicated at 66, as well as an inwardly disposed shaft 68 of aheat source generally indicated at 70. In addition to the shaft 68, theheat source 70 includes a hot gas source 72, which projects hot gasesinto an enclosure 74 surrounding the lower end 68a of the shaft 68. Fins76 aid in transferring heat from the gas to the shaft 68. The shaft,which is of a material having a high heat conductivity, in turn conductsthe heat to the cylinder 64.

The cylinder, in absorbing heat from the shaft 68, undergoes an increasein temperature, with a resulting radial expansion away from theshaft'until it engagesthe heat sink 66. The latter conducts heat awayfrom the cylinder and disposes of it with the aid of fins 78. Thecylinder 64 then cools and returns to the shaft 68, whereupon the cycleis again repeated.

More specifically, the cylinder 64, which is of a pyroelectric material,has inner and outer electrodes 80 and 82, preferably taking the form ofhighly reflective silver coatings to minimize transfer of heat into andout of the cylinder by radiation. The material of cylinder 64 is any oneof a variety of well known ferroelectric ceramic materials havingpiezoelectric properties. Examples of such materials are lead zirconate(lead titanate, barium titanate, calcium titanate, and variouscombinations of these. Leads 84 and 86 connect the electrodes 82 and 80to the ends of a potentiometer 88. The tap 89 of the potentiometer isconnected to the control electrode of a switch 90, which is in serieswith a load 92 across the leads 84 and 86. A capacitor C is connectedbetween the tap 89 and the lead 84. The switch may take theform of athyratron or a silicon controlled rectifier. Thus, once the voltage atthe control electrode reaches a firing potential, the switch conductsand remains in that condition until the voltage of the source switchedthereby drops 'below a certain minimum level.

During operation, the cylinder 64 is centered with respect to the shaft68 and heat sink 66 by a pair of rings 94 and 96 disposed between thecylinder 64 and shoulders 98 and 100 on the heat sink. The rings 94 and96 are of highly resilient material such as silicone rubber, capable ofwithstanding the temperature to which the cylinder 64 is subjected.

In FIGURE 1, the relative dimensions have been exaggerated for the sakeof clarity. In practice, the cylinder 64 may have a thickness of 50mils, a length of several inches and a radius of the same order ofmagnitude. When the cylinder is contracted against the shaft 68, thespacing between the outer electrode 82 and the heat sink 66 may be ofthe order of 50 mils.

In considering operation of the generator of FIGURE 1, assume that atemperature of the cylinder 64, slightly greater than that of the heatsink 66, corresponds to a shrink-fit of the cylinder on the shaft 68.Conversely, a cylinder temperature slightly less thanthat of the shaft68 corresponds to a similar fit between the cylinder and the heat sink.If, at a low temperature, the cylinder 64 is suddenly drawn into contactwith the shaft 68, conduction of heat from the shaft to the cylinderwill result in a rapid temperature rise in the latter. We may assumethat the polarization in the cylinder 64 is such that the primary andtertiary pyroelectric effect accompanying the temperature rise cause thelead 84 to become positive with respect to the lead 86.

With the extreme thinness of the cylinder, the temperature rise is veryrapid, so that the combined wave form of'the primary and tertiaryvoltages is similar to a fairly sharp pulse. This pulse is readilypassed by the capacitor C to the tap 89 and control electrode of theswitch 90. The switch is rendered conducting, thereby connecting theload 92 to the lead 84. The load 92 has an impedance which issubstantially less than the impedance of the cylinder 64 as definedbelow, and, therefore, the series combination of the switch 90 and load92 acts essentially as a short circuit across the electrodes 80 and 82.

The short circuiting of a pyroelectric transducer greatly retards thechange in dimension thereof resulting from temperature changes, and,thus, by the time the cylinder 64 has developed sufiicient internalstresses to recover from its shrink-fit and begin to expand away fromthe shaft 68, it has reached a temperature almost equal to that of theshaft. This temperature, as pointed out above, is great enough toprovide expansion all the way to the heat sink 66 and, in addition,effect a compression fit against the heat sink. The energy from thefirst and third pyroelectric effects has by now been dissipated in theload 92, and the switch 90 has accordingly opened to disconnect the loadfrom the lead 84.

With the load 92 disconnected, the cylinder 64 expands rapidly and thesecondary pyroelectric effect (piezoelectric effect) comes into play.This voltage .is ultimately substantially greater than the primary andtertiary voltages, but its rate of increase is materially less, and,therefore, it is not bypassed to the tap 89 by the capacitor C. Thus,the voltage at the tap 89 depends on the setting of the tap as well asthe voltages at the lead 84 with respect to ground. The tap 89 is set sothat its voltage will reach the firing level of the switch 90 justbefore the cylinder 64 contacts the heat sink 66. At this point theswitch once again conducts, to discharge the secondary energy into theload 92; the switch then opens, just as contact is made between thecylinder and heat sink.

Then the reverse process begins, except that this time the polarity ofthe primary and secondary voltages is reversed. The switch 90 conductsinitially to help delay contraction of the cylinder 64 and then shutsoff to permit rapid contraction thereof. Just prior to contact with theshaft 90,'the switch is turned onto deliver secondary energy to the lead92.

The expansion and contraction of the cylinder 64 thus continuesindefinitely. The rate at which this occurs can be quite rapid. In fact,a rate of 40,000 cycles per second may be desirable for a unit havingthe above dimensions. By suitable choice of radius, this may be made tocorrespond to the natural frequency of the cylinder 64 in the radialmode, and the resulting resonance aids in effecting the desired motion.

The impedance of the cylinder 64 may be roughly defined as thecapacitive reactance at the frequency of operation. As pointed outabove, the impedance of the load 92 should be substantially less thanthe impedance of the cylinder at the effective frequency of theprimarytertiary voltage spike. More importantly, it should also besubstantially less at the cyclic operating rate of the 4- generator.This will insure essentially complete discharge of the piezoelectricenergy to the load 92.

The impedance of the load 92 should, of course, be substantially greaterthan that of the switch 90 when the latter is conducting, so thatsubstantially all of the power developed in the cylinder 64 isdissipated in the load.

In many cases, the cylinder 64 will be so thin that the resulting highcapacitance thereof will provide-a fairly low impedance. It' may then bedifiicult to provide a load impedance which is substantially lower thanthe impedance of the cylinder. In such cases, one may resort to theconfiguration of FIGURE 2, in which the cylinder 64 is schematicallydivided into a series of arcuate segments 64a-64h. These segments mayagain be radially polarized, but with polarization in the oppositedirection in adjacent segments. Thus, in the segment 64a, thepolarization may. be in the outward direction, and, in segments 64b and64h, it will then be inward. The electrodes 80 and 82 are alsosegmented, with an inner electrode segment 80a covering the cylindersegments 64a and 64b, an overlapping outer segment 82]) on the segments64b and 640, a further overlapping segment 80b and so on around thecylinder 64.

Thus, it will be seen that between the leads 84 and 86, connected to theouter electrode segments 82c and 82a, there is a capacitance comprisingthe capacitance of the individual segments in series. Furthermore, thevoltage developed between the electrodes 82a and 80a adds to the voltagedeveloped between the segments 80a. and 82b, and so on around thecylinder. The increase in output voltage is offset by the decrease incapacitance, so that the total power developed by the cylinder 64 isunchanged. In the illustrated embodiment, the composite capacitor iseight times as thick as the cylinder 64 of FIGURE 1, and, furthermore,the effective area of each segment is one-eighth the area of thecapacitor in FIG- URE 1. Accordingly, the capacitance is reduced by afactor of 1/64.

It will be appreciated that other circuit elements than the onesspecifically shown in FIGURE 1 can be used in accomplishing thefunctions set forth above. For example, a capacitor (not shown)connected between the lead 84 and load 92 may be used together with, orin place of, the capacitor C.

Moreover, different modes of operation are within the contemplation ofour invention. Thus, the use of a switch may be dispensed with and thecylinder 64 connected di-' rectly to the load 92. However, in that casecare must be taken in selecting the various parameters affectingoperation, or the cylinder may come to an equilibrium position betweenthe shaft 68 and heat sink 66, with a complete cessation of operation.Also, the constant connection to the load will slow down operationsomewhat, with a correspondingly reduced power output.

It will be apparent that two important advantages of the system shown inFIGURE 1 are the absence of wearproducing motion and the omission ofsources of motion external to the pyroelectric transducer.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efiiciently attained and,since certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawing shall be interpreted as illustrative and not in a limitingsense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

We claim:

1. An electrical generating system comprising, in com- 4 bination, apyroelectric transducer having the form of a shell, said transducerhaving a pair of electrodes and being capable of developing a voltagebetween said electrodes in response to changes in its temperature, aheat source disposed within said shell and conforming to a substantialportion of the inner surface thereof when at a first temperature, a heatsink disposed adjacent an outer surface of said shell opposite saidinner surface and substantially conforming to said outer surface when ata second temperature greater than said first temperature, whereby saidshell cyclically expands and contracts to alternately absorb heat fromsaid source and transfer it to said sink.

2. The combination defined in claim 1 including an electrical load andmeans connecting said load between said electrodes.

3. The combination defined in claim 2 including means for connectingsaid load across said electrodes when said shell contacts said sourceand said sink, disconnecting said load from said electrodes during asubstantialportion of the interval after said shell loses contact withsaid source and said sink and before it again contacts one of them andconnecting said load to said electrode immediately prior to contactbetween said shell and said source and sink.

4. The combination defined in claim 2 in which the impedance of saidload is substantially less than the impedance of said shell.

5. The combination defined in claim 1 in which the surfaces of saidshell facing said source and said sink and the surfaces of said sourceand sink facing said shell are of a highly reflective material.

6. An electrical generating system comprising, in combination, apyroelectric transducer having a pair of electrodes, said transducerbeing capable of undergoing changes in dimension and developing avoltage between said electrodes in response to changes in itstemperature, a heat source and a heat sink, said transducer beingarranged so that its dimensional characteristics with respect totemperature position it in relatively close heat exchange relationshipwith said source when at a first temperature and relatively close heatexchange relationship with said sink when at a second temperaturegreater than said first temperature, said first temperature being atleast as great as the temperature of said sink and said secondtemperature being no greater than the temperature of said source,whereby said transducer cyclically comes into close heat exchangerelationship with said source and said sink.

7. The combination defined in claim 6 including an electrical load andmeans connecting said load to said electrodes.

8. The combination defined in claim 7 including means for connectingsaidload to said electrodes when said transducer is in positionscorresponding to close heat exchange relationship with said source andsink, thereby to retard changes in dimension of said transducer,disconnecting said load during substantial portions of the intervalswhen said transducer is between said positions and connecting said loadto said electrodes immediately prior to arrival of said transducer atsaid positions.

References Cited by the Examiner UNITED STATES PATENTS 2,838,723 6/1958Crownouer 310 9.1

LLOYD MCCOLLUM, Primary Examiner.

A. H. TISCHER, Assistant Examiner.

1. AN ELECTRICAL GENERATING SYSTEM COMPRISING, IN COMBINATION, APYROELECTRIC TRANSDUCER HAVING THE FORM OF A SHELL, SAID TRANSDUCERHAVING A PAIR OF ELECTRODES AND BEING CAPABLE OF DEVELOPING A VOLTAGEBETWEEN SAID ELECTRODES IN RESPONSE TO CHANGES IN ITS TEMPERATURE, AHEAT SOURCE DISPOSED WITHIN SAID SHELL AND CONFRONTING TO A SUBSTANTIALPORTION OF THE INNER SURFACE THEREOF WHEN AT A FIRST TEMPERATURE, A HEATSINK DISPOSED ADJACENT AN OUTER SURFACE OF SAID SHELL OPPOSITE SAIDINNER SURFACE AND SUBSTANTIALLY CONFORMING TO SAID OUTER SURFACE WHEN ATA SECOND TEMPERATURE GREATER THAN SAID FIRST TEMPERATURE, WHERE-